They said it was a gimmick, but that isn't true. Kristin Scott Thomas and Lia Williams are swapping roles, from one night to the next, in Ian Rickson's staging of 'Old Times' at London's Harold Pinter Theatre. As some cynics have observed, this sounds like a canny ruse to encourage twofold box-office bookings. In fact, this is a superb, cleverly apt production and if you go to see it twice – to witness both casting configurations – Pinter's slippery script becomes even more richly intriguing.

In this darkening three-hander, Deeley (a documentary-maker played by Rufus Sewell) and his wife, Kate (Scott Thomas on my first viewing), are waiting in their coastal home for a visit from her erstwhile friend, Anna. Elegantly curled on a settee, in blood-red velvet slacks – with a remote look, as if in a world of her own – Scott Thomas says she has not seen Anna for 20 years. Sewell's initially smirking Deeley learns that the duo, as young women, shared digs in west London, and Anna was prone to steal the other's underwear.

When Anna shows up (Williams, lithe in turquoise and so wraith-thin she almost disappears sideways on), the trio's reminiscences become increasingly tense, with riptides of sexual attraction coursing under the surface, brooding hostilities and power games. The memories recounted, moreover, become disconcertingly unreliable. Anna declares she was present at the film house where Deeley romantically recalls first meeting Kate. He, in turn, claims to have previously picked up Anna in a seedy pub and freely gazed up her skirt at a party.

Facts and confabulations become disturbingly hard to disentangle. Is Sewell, skulking in the shadows when the two women start enacting the past, disempowered or dreaming up erotically twisted fantasies-within-fantasies? Ultimately, he no longer seems sure which woman he met when, and this production is even more mind-bending when you view it again, with Williams as Kate and Scott Thomas as Anna. They switch their hair colour but only some of their clothes, playing some exchanges fractionally differently, some startlingly so. Actually, I suspect seeing this 'Old Times' multiple times could become an obsession.